Color crt assembly



v Aug. l2, 1969 a. G. MARKS coLoa om ASSEMBLY Filed June 14, 1968 has@ AfTOR/Yfy UnitedStates Patent() COLOR CRT ASSEMBLY Bruce G. Marks, Lansdale, Pa., assignor to Philco- Ford Corporation, Philadelphia, Pa., a corporation of Delaware Filed June 14, 1968, Ser. No. 737,007

Int. Cl. H01j 29/50 U.S. Cl. 315-13 10 Claims ABSTRACT OF THE DISCLOSURE Color cathode ray tube assembly employing three electron guns, each with an einzel type electron focus lens, each lens employing lirst and third electrodes maintained at second anode potential and second (intermediate) electrode maintained at cathode potential. A purity magnet assembly is mounted around neck of tube forward of the second electrodes of the electron guns and to -the rear of the convergence yoke, and a blue lateral magnet assembly is mounted on the neck of the tube forward of the convergence yoke.

This invention relates to color cathode ray tubes of the plural electron gun, shadow mask variety, and more part-i'cularly to such a tube employing improved electron guns with an improved 'arrangement of the -beam position correction magnets therefor.

Prior art The typical cathode ray tube (CRT) presently employed in home col-or television receivers has a faceplate backed with a layer ofcolor phosphor dots arranged in groups of three, wherein the respective dots of each group are emissive of red, green, and blue light when activated by an electron beam. Behind the 'color phosphor layer (which may 'be backed by a 'layer of aluminum) is mounted a shadow mask, which is a lforaminous metallic screen having holes arranged so that a hole is behind the center of each group of phosphor dots. Mounted in the neck or rear of the `tube are three electron guns which are positioned symmetrically around the central laxis of the tubes neck. Each gun is aimed so that its electron beam will pass through the holes in the shadow mask at such an angle that it will impinge on phosphor dots of a particular color only. Each gun Iis designated by the color of the llight emitted by the phosphor dots upon which its beam impinges; hence in a typical tube there is a red gun, a green gun, and a blue gun.

Each electron gun consists of the following elements, progressing from the rear to the front of the tube: a heating filament, a cathode which is heated by the filament for emission of electrons, ya control electrode (commonly designated G1) for controlling the number of electrons traveling toward lthe `front of the tube from the cathode, a second or accelerating electrode (G2)for attracting the emitted electrons, and third and fourth electrodes (G3 and G4) for further accelerating andl Ifocusing or `collimating the electrons so that a narrow beam of 'electrons will be directed toward the faceplate of the tube. The inside of the faceplate and side walls of the tube are conductive and constitute a fth electrode, called a second anode.

In a typical tube having a picture which measures diagonally in lthe range of 17.to 23., the control electrode, G1, is usually biased at a negative voltage about 50 volts below the cathode voltage, the initial accelerating electrode, G2, is usually biased several hundred volts more positive than the cathode, and `the second anode is biased about 25 y,thousand volts 'more positive than the Icathode. The first of the two focus electrode-s, G3,is usually biased ice about 4 thousand volts more positive than the cathode, and the second focus electrode, G4, is `maintained at a iinal accelerating voltage equal to the screen potential (25 kv.). The *focus and accelerating electrodes are spaced from each other and maintained at diterent voltages in order to create electrostatic tields therebetween which act as electron lenses which collimate the electron beam. v

Mounted around the neck of the tube are several magnetic devices for adjusting the positions of the electron beams. Starting backward from the front of the tubes neck, a deflection yoke is mounted around the front of the ne'ck'next to the funnel portion of the tube; the magnetic ields from this yoke cause the electron beam to scan across the tube in a series of adjacent horizontal paths. Mounted around the neck to the realof the deflection yoke is a convergence yoke assembly; this assembly includes Ipermanent and electroma'gnets which adjust the static and dynamic radial positions of the electron beams so that all three beams will converge, i.e., impinge at substantially the same locus as they scan across the Iface of the tube. Associated with the convergence assembly within the neck of Ithe tube are six radial pole pieces (two for each beam) which concentrate the magneti'c force from the convergence assembly in lateral directions so that the positions of the electron beams will be shifted in a radial direction as desired. Mounted yaround the neck of the tube behind the convergence assembly is a blue lateral permanent magnet assembly for adjusting the lateral position of principally the blue-phosphor excting beam as a further means of obtaining proper convergence.

In order for each electron beam to excite only phosphor dots of its particular color, it is necessary that each beam approach the shadow Imask at the proper angle. It is theoretically possible to -attain this objec-tive by mounting the three electron guns in certain precise positions in the neck of the tube. However since it is impossible to attain these precise mounting positions in a mass production operation, a purity magnet assembly is employed to correct the positions of the electron beams as necessary so that they will approach the shadow mask at the proper angle. rl`he purity magnet assembly usually is mounted around the tubes neck over the space between the G2 and G3 electrodes, to the rear of the blue lateral magnet assembly.

One difliculty associated with the aforede-scribed CRT assembly is the requirement of three different voltages which must be supplied to the accelerating and focusing electrodes of the electron guns. As mentioned before, an initial accelerating voltage of about 400 volts must be supplied to the accelerating electrode (G2), a focus voltage of about 4000 volts must be supplied to the rst focus electrode (G3), and the iinal accelerating voltage (25 kv.) must be supplied to the second focus electrode (G4). The provision of the rst and third of the foregoing voltages presents no diticulty since the 400 v. accelerating voltage may be obtained `from the B+ voltage source found in the chassis of the receiver and since the 25 kv. nal accelerating voltage exists at the second anode of the tube.

The need for a 4 kv.- focus voltage, however, creates diiiculty since no such voltage exis'ts at any other point in the tube or on the chassis. Hence this voltage must be specially generated. This is usually done by providing an additional winding on the horizontal output transformer of the associatedreceiver, together with appropriate circuitry, including a rectifier, lter, and limiter. It will be appreciated by those skilled in the art that it would be highly desirable from the standpoints of cost, reliability, weight, and ease of servicing if this additional supply source could be eliminated. Einzel type electron guns, which do not require this additional supply Source, thave not been used because they produced electron beams wlhose spot size at the faceplate of the tube was excessively large. The vpresent invention provides a CRT assembly which does not require a separate focus supply source, yet does not require any sacrifice in performance or increase in electron beam spot size at the faceplate of the tube.

Objects Accordingly several objects of the present invention are: (1) to provide a new and improved color cathode ray tube assembly, (2) to provide a color cathode ray tube assembly which does not require a separate focus supply voltage, and (3) to provide a color cathode ray tube assembly which will endow its receiver with increased reliability, decreased weight, decreased cost, and increased ease of servicing.

Summary mounted forward of the convergence yoke assembly and behind the deection yoke.

Drawings FIG. l shows a diagram of the color CRT assembly of the present invention and FIGS. 2(a) and (b) show purity and blue lateral magnets of the type used in such assembly and the effects of their elds on electron beam positions,

Description of Assembly FIG. l shows the improved color CRT assembly of the present invention in partial cutaway, partial cross sectional and partial schematic form. The tube comprises an evacuated glass envelope having a neck portion 10, a funnel portion 12 (which is mainly omitted in the drawing to save space), and a faceplate portion 14. Deposited on the inside of the faceplate 14 is a layer of color phosphor dots 16 which may be backed by an aluminum layer. Mounted behind phosphor layer 16 is a foraminous metallic shadow mask 18. The inside of the funnel shaped portion 12 and the front of the neck portion 10 are covered with a conductive coating 20, such as a graphite preparation of the type sold under thev trade name Aquadag Aquadag coating 20 is in contact with the aluminum covered phosphor layer 16 so that the entire interior surface of the faceplate as well as the funnel portion 12 of the tube constitute a second anode. A 25 kv. source 22 is connected to the Aquadag coating to maintain the second anode at this potential.

Mounted within the neck 10 of the tube are three electron guns which are symmetrically positioned at the apices of an imaginary equiangular triangle whose plane is normal to the axis of the tube. The tube is viewed at an angle such that two of the electron guns, 26 and 28, are equidistant from the observer; hence the third gun (not illustrated) will lie midway between and behind one of the two guns illustrated. The top gun 26 is shown in section and the bottom gun 28 is shown in full view. Since the elements 'of eaoh gun are similar, they are designated with identical reference numerals, with the numerical designations for the bottom gun 28 being primed.

Each gun includes a filament 30 for heating a surrounding cylindrical cathode 32 which is closed at one end by a wall whose exterior surface contains an electron emissive coating. Mounted generally around cathode 32 to the front thereof is a cylindrical control electrode G1 which has one end thereof closed by a wall having a central opening therein to permit electrons to pass therethrough.

Spaced from the cathode is a cylindrical accelerating electrode VG2 which is closed at its rear'end'by a similar wall having a similar opening therein to permit the passage of electrons therethrough, whereby the partially closed end of electrode G2 faces the partially closed end of electrode G1. The front end of electrode G2, as well as the ends of most of the remaining electrodes, are curved backwardly over' the'outside of the cylinder as illustrated in order to present a rounded surface to the end of the adjacent electrode so as to obviate arcing.

The next electrode G3 is a focus and accelerating electrode which consists of a relatively narrow rear cylindrical portion joined to a relatively wide front cylindrical portion by a substantially radial wall member as illustrated. TheY rear end of the narrow portion is positioned inside the front end of electr-ode G2. The following electrode G4 is cylindrical focus electrode which is open at both ends and which has a shorter axial length than electrode G3. The last focus electrode G5 has a cylindrical rear portion whose rear end is flared and curved away from electrode G4. The front portions of all three G5 electrodes are joined together in a conventional unitary assembly which contains radial pole pieces (not shown) and sectors (not shown) for isolating each set of pole pieces from the other sets. Attached to each of the G5 electrodes is an interconnection member such as 34 which electrically connects the G5 electrode assembly with the Aquadag coating 20.

At the rear end of the neck of the tube are connection pins 36 from which the required bias and signal voltages are supplied to the various electrodes in the electron guns by means of rigid internal connection leads which are not illustrated.

The 4filaments 30 are `connected to the usual heater voltages, eg., 6.3 volts AC. The cathodes 32 are biased at a reference potential which may be different than the reference potential employed in the circuitry of the color TV receiver but which is `designated in the present drawing by a ground symbol. The cathode 32 may also have supplied thereto a signal voltage, such as chrominance information. If this is the case a suitable load impedance (not shown) is provided between the cathode and the point of reference potential. The `control electrodes G1 are biased at a negative potential of about 50 volts and may have luminance signal information supplied thereto. The accelerating electrodes G2 are biased at a positive voltage which in a typical 23 inch tube may be about +400 volts. The first electrodes G3 are biased at the potential of the G5 electrodes by means of the illustrated interconnection lead 35 between G3 and G5. As explained later, in 4a typical tube these electrodes are maintained at a potential of over about ten thousand volts, typically 25,000 volts. The second or intermediate focus electrodes G4 are biased at substantially the `cathode potential. Due to the great dilference in potential ybetween electrodes G4 and their surrounding electrodes G3 and G5, it will be understood that biasing electrodes G4 at substantially the cathode potential may contemplate the biasing of electrodes G4 to a potential several hundred volts different lfrom the cathode voltage, e.g., to the voltage of the G2 electrodes (+400 volts). The third focus electrodes G5 are biased at the second anode voltage (+25 kv.) by means of the interconnection members 34.

The focus electrodes, G3, G4, and G5, constitute a three-electrode electron lens which is termed an einzel (i.e., single) lens since its optical homologue is a single lens surrounded on both sides by media of the same refractive index.

Mounted around the Lfront of the neck portion of the tube is a conventional deec-tion yoke 24 which is arranged to provide magnetic @fields which cause the electron beams traveling from the neck to the faceplate to scan across the faceplate of the tube in response to horizontal and vertical scanning or deflection signals.

Also mounted around the neck of the tube behind deection yoke 24 opposite the forward portion of electrode G5 is a convergence yoke assembly 36 which consists of three permanent magnets and three associated electromagnets which have pole pieces (such as shown at 38) for coupling magneticvforce to the radial pole pieces (not illustrated) inside -the `forward portion of electrodes G5.

Mounted around the neck of the tube to the front of convergence yoke 36 and adjacent thereto is a blue lateral magnet assembly 40. While the blue lateral assembly 40 is illustrated as symmetrically positioned around the tubes main axis, it is actually asymmetrically positioned (for a reason described infra) so that its center lies slightly below the tubes main axis. Mounted around the neck to the rear of convergence asem'bly 36 and ahead of the space between electrodes G4 and G5 is a purity magnet assembly 42.

While the deflection yoke, convergence yoke assembly, blue lateral magnet assembly, and prity magnet assembly are all conventional, one each of the purity and blue lateral magnets is illustratedinvFIGS. 2(a) and (b) in order to pro-vide a fuller understanding of the invention.

The purity magnet assembly consists of two ring magnets of the type shown in FIG. 2(11) positionedtogethe-r as illustrated at 42 Iin FIG. 1. Each magnet has a flat annular shape and is magnetized to provide north and south poles at opposite ends of the circumference thereof as indicated. A magnetic eld is created inside the neck of the tube by this assembly, as indicated by the dashed lines. As is Well known, the [magnitude and orientation of this teld can be adjusted by rotating one or both of the magnets as desired. The generally lateral shift-s of beam positions by a lfield having a vertical orientation are illustrated by the broken and full circles which represent, respectively, the beam positions without and with magnetic purity field applied. v

The blue lateral magnet assembly Consists of two adjacent flat Itoroidal members of the type shown in FIG. 2(b) which are polarized to have three north and th-ree south poles on the circumference thereof as illustrated. The orientation of the iields produced inside the neck of the tube by -asymmetrically-posit'ioned the blue lateral magnet assembly` with respect to the electron beams are also illustrated by broken lines. The orientation and magnitude of these fields can be adjusted by rotating either or both of the blue lateral magnets as desired. It should benoted that the 'fields `from-this magnet are such as to shift the position of `the top beam (which in practice is the blue beam) in one lateral direction and the positions of the other (green and red) beams in the opposite direction to a lesser extent.

Operation of assembly In operation, electrons emitted by each of the cathodes 32 are controlled, accelerated, and collimated in conventional fashion by electrodes G1 to G5 so that three narrow electron beams impinge on the faceplate of the tube. The dynamic magnetic field produced by deflection yoke 24 causes the electron beams to scan across the faceplate of the tube in conventional fashion. The permanent magnets in convergence assembly 36 are adjusted and suitable signals are ysupplied to the electromagnets of theconvergence assembly `so that the radial positions of the beams are corrected to cause beams to converge as they are swept across the -face of the tube. Additional static lateral convergence correction is provided by the blue lateral 4magnet assembly 40 'which shifts the lateral position of the blue beam `in the desired direction and the position of the red and ygreen beams to a lesser extent in the'opposite direction so that the blue beam converges with Ithe red and green beams. The position of lall three beams is shifted, generally laterally, by the field from the purity magnet assembly 42 so that all three beams will approach the shadow mask 18 at the proper angle,

whereby each beam excites only phosphor dots of a vsingle color.

The einzel lens consisting of electrodes G3, G4, and G5 provides two powerful electrostatic elds, one between electrodes G3 and `G4 and the other between electrodes G4 and G5. The iirst `of these iields is divergent and the second is convergent. The overall effect of both lenses is to collimate and focus the electron beams so that they have minimum spot size when -they impinge on the face plate of the tube.

While the einzel lens eliminates `the need Afor a separate focus voltage supply source in the neighborhood of 4 kv., it has been discovered that unless the electron beams pass through the electrodes of the einzel lens on the center axis thereof, the beams will become greatly enlarged in size and will produce 'a large spot on the faceplate of the tube which results in poor focus (blurred images). For this reason it has not been considered possible to use an einzel type lens in a shadow mask type tube. According to the pre-sent invention, the beams are not diverted from the center axes of the guns since `the purity :magnet assembly 42 and blue lateral magnet assembly 40, which would so ldivert the beams as illustrated in FIG. 2 i-f positioned over or behind the lenses, are positioned -ahead of the lenses. Thus according to t-he teachings of the present invention, an einzel lens can be used in a color CRT without increase in electron beam spot size.

The illustrated positions of the purity and blue lateral magnets are in fact relatively critical. As stated, it is not feasible to position either the purity or the blue lateral magnet assembly over or behind the einzel lenses (i.e., over to the rear of the gap between G4 and G5) without serious degradation of spot size. Also, it is not feasible to position the purity and blue lateral magnet assemblies close to or in front of deection yoke since at such location a much stronger magnet is required whose eld produces an unacceptable amount of magnetic distortion of the electron beams. According to the invention the purity magnet is positioned forward of lche rear end of the G5 electrode, preferably to the rear of the convergence yoke and adjacent the rear end of the G5 electrode. The blue lateral magnet assembly is positioned between the convergence and deection yokes, preferably adjacent the convergence yoke.

Previous color TV tube assemblies employed noneinzel type electron guns with the purity magnet assembly positioned near the rear of the neck of the tube, i.e., in the vicinity of G2. In the present assembly the purity magnet assembly is located toward the front of the tubes neck where theoretically a greater amount of ux is required to provide an equivalent amount of correction of the electron beam. However it bas been found, quite unexpectedly, that stronger purity magnets are not required in the present position since the fringing flux to the rear of the purity magnet assembly, which did not intercept the electron beams in prior art arrangements, does intercept the beams and hence is utilized in the present arrangement. In fact, the illux requirement for a purity magnet assembly positioned as indicated is about 25% less than prior arrangements due to the utilization of this fringing flux.

It has been determined experimentally that for a tube assembly with typical convergence and purity adjustments, a 35% reduction in electron beams spot size is provided for an einzel gun assembly when the purity and blue ylateral magnet assemblies are positioned as indicated in FIG. l vis-a-vis prior art positions. In addition, since the purity magnet assembly, convergence yoke assembly, and blue lateral magnet assembly are all relatively close together, all three of these elements can easily be assembled in a unitary structure, thereby further reducing assembly and service costs.

While there has been described what is at present considered to be the preferred embodiment of the invention it will be apparent that various modifications and other embodiments thereof will occur to those skilled in the art within the scope of the invention. Accordingly, it is desired that the scope of the invention be limited by the appended claims only.

I claim:

1. A color cathode ray tube assembly,

said assembly including a cathode ray tube having a rear neck portion and a fiared front portion with a faceplate covering the wide end thereof,

said neck portion including a plurality of electron guns for producing a corresponding plurality of electron beams directed toward said faceplate,

each of said guns including at least the following spaced electrodes, progressing toward the front of said t-ube: a cathode, a cylindrical control electrode, and three cylindrical focus electrodes,

means for biasing said control electrode at a negative voltage with res-peet to said cathode, means for biasing said faceplate at a potential at least ten thousand volts more positive than said cathode, means for biasing the first and third of said focus electrodes at substantially the potential of said faceplate, means for biasing the second of said focus electrodes at substantially the potential of said cathode,

a magnetic deection yoke around the front of said neck portion for producing magnetic fields which cause said electron beams to scan across said faceplate,

magnetic convergence means around said neck portion,

forward of said guns and to the rear of said deflection yoke, for adjusting the radial positions of said beams, and

magnetic color purity correction means around said neck portion, to the rear of said convergence means and forward of the gap between the second and third of said further electrodes, for adjusting the position of at least one of said beams.

2. The combination of claim 1 wiherein said purity means comprises at least one annular magnet encircling said neck portion.

3. The combination of claim 1 further including magnetic means around said neck portion to the rear of said deflection yoke and forward of said convergence means for adjusting the lateral position of one of said beams in relation to at least another of said beams.

4. The combination of claim 1 wherein said guns are three in number and are spaced at the apices of an imaginary equiangular triangle which lies in a plane normal to the axis of said tube, each of said guns also including a cylindrical accelerating electrode between said control electrode and the first of said three focusing electrodes.

5. The combination of claim 1 wherein each of said guns also includes a cylindrical accelerating electrode between said control electrode and the first of said three further electrodes, said accelerating electrodes being biased at a potential intermediate the potential of said cathodes and said faceplate, the intermediate one of said three further electrodes of each of said guns having a shorter length along the axis of said t-ube than the first and third of said three further electrodes, said color purity correction means comprising at least one annular magnet mounted around said neck portion over the third iof said three further electrodes and adjacent the rear end thereof.

6. In a color cathode ray rtube assembly of the type including:

a cathode ray tube having la faceplate whose rear surface is covered with triple-ts of phosphor dots of three different colors,

la forarninous metallic shadow mask mounted in said tube behind said faceplate, `and three electron beam-producing guns in the neck of said tube spaced around an axis thereof,

each Iof Isaid guns including a cathode, a cylindrical con-trol electrode biased at a negative potential with respect to said cathode, a plurality of spaced cylindrical focus electrodes mounted within said neck, Ione of said plurality of electrodes Abeing maintained at a potential 4at least ten thousand vol-ts higher than said cathode,

a defiec'tion yoke mounted around the front of the neck of said tube for causing said electron beams to scan across the face of said tube,

convergence means mounted around the neck of said tube, behind said yoke, for changing the radial positions of said beams, and

color purity correction means mounted around the neck of said tube for changing the position of said beams;

the improvement wherein:

said plunality of electrodes comprises first, second, and

third electrodes, `the first and third rbeing biased substantially at the potential of said faceplate, the second being biased at substantially the potential of said cathode, said purity ymeans being mounted around the space between the rear -of said third electrode and said deflection yoke.

7. The assembly of claim 6 further including lateral beam position correction means around said neck forward of said convergence means and to the rear of said yoke for adjusting the lateral position of at least one of said beams with respect to Ithe others of said beams.

8. The `assembly of claim 6 wherein said color purity correction means comprises at least one annular magnet having a north pole and a south pole on the circumference thereof.

9. The assembly of claim 6 wherein each of said guns further includes an additional annular accelerating electrode spaced 'between said `control electrode and the first of said plurality of electrodes, said additional electrode ibeing biased at a potential intermediate the bias potential of said cathode and said faceplate.

'10. The assembly of claim 7 further including lateral beam posi-tion correction means around said neck to the rear of said `deflection yoke and to the front of said convergence means for 'adjusting the lateral position of at least one of -said beams with lrespect 'to the `others of said beams, and wherein the second of said additional elec-trodes of each of said guns has a shorter length along the axis `of said tube than the first and third of said addi-tional electrodes.

References Cited UNITED STATES PATENTS 2,898,493 8/1959 Burdick 315-13 X 3,076,121 l/l963 Stone 315-31 X 3,294,999 12/1966 Van Hekken 315-13 X 3,295,001 12/1966 Burdick et al. 313-82 RODNEY D. BENNETT, I r., Primary Examiner M. F. HUBLER, Assistant Examiner U.S. C1. X.R. 

